Vitamin B12 is an essential nutrient for humans, that is only synthesized by microbes. Little is known about its biosynthesis or its physiological importance to the microbes that produce it. The bacterium Salmonella typhimurium is capable of a complete synthesis of B12, but does so only under anaerobic conditions. Since sophisticated genetics can be done in this bacterium, we propose a genetic approach to understanding the biosynthesis of vitamin B12 in this organism. We will identify the genes involved (probably an excess of thirty) and will pursue how this pathway is regulated with special emphasis on how 02 is involved. The genetic and biochemical study of enteric bacteria has been done largely under aerobic conditions. Relatively little is known about the details of the anaerobic life-style of these organisms even though virtually all their interactions with human hosts (colonization and pathogenesis) proceed in the absence of oxygen. We suspect that production of B12 anaerobically indicates the existence of a substantial body of metabolic activity that occurs only anaerobically and is totally unknown. We propose to approach these aspects of metabolism by studying the role of B12 in the anaerobic life of Salmonella.